1. Field of the Invention
The present invention relates in general to an antipollution system and more particularly to an evaporative emission control system of a motor vehicle.
2. Description of the Prior Art
Nowadays, motor vehicles are equipped with an evaporative emission control system which prevents the escape of fuel (gasoline) vapors from the fuel tank and carburetor, whether or not the engine is running. Usually, the system uses an activated charcoal canister to trap the vapors when the engine is shut off. On restarting, flow of filtered air through the canister purges the vapors from the charcoal, and the mixture goes through purge tubes into the carburetor to be burned in the engine.
Some of the conventional systems, however, do not take a satisfied measure to deal with a saturated condition of the canister. In fact, once the activated charcoal of the canister is saturated in adsorbing the fuel vapors, very rich mixture (or vapors) is produced in the purge tubes or lines thereby causing the engine air-fuel mixture to become greatly rich. This phenomenon not only deteriorates the exhaust characteristics but also lowers the fuel economy of the engine. This drawback will be well understood from an after description where one of the conventional emission control systems is described.